The measurement of blood flow and blood velocity in vessels is an important diagnostic tool for determining blockage in the vessel as well as other vascular disorders. Measurement of retinal blood flow is particularly valuable for providing information relating to circulation of blood through the numerous submicron size blood vessels in the eye. For example, retinal vascular disorders can be diagnosed by monitoring blood flow through the retina. Accurate measurement of retinal blood flow is also essential in monitoring the effectiveness of various drug treatment methods for retinal vascular disorders.
In the past, a number of methods of measuring vascular blood flow and in particular retinal blood flow have been developed. These methods often use a fluorescent dye, such as fluorescein, which can fluoresce when exposed to light within a narrowly defined wavelength. The fluorescent dye is injected into the blood stream in a predetermined site. A sharp, easily visible wavefront of the dye, referred to as a bolus, is obtained by controlling the injection of the dye in the vessel. A bolus cannot be obtained repeatedly because the previously injected dye accumulates in the vessel and causes background interference thereby preventing precise identification of the bolus. Another disadvantage of injecting the dye directly into the bloodstream is that the concentration of the dye is diluted as the dye passes through the different vessels. As the dye is diluted, the bolus is more difficult to detect. Furthermore, a bolus in microcirculation, such as that in the optic nervehead, cannot be readily detected.
Efforts have been made to overcome some of the deficiencies of injecting dyes directly into the bloodstream. One method includes the use of lipid vesicles, known as liposomes, to encapsulate the dye. The lipid vesicles have also been used to encapsulate drugs. The lipid vesicles can be injected into the bloodstream where they rupture to release the encapsulant. To control the release of the encapsulant, designated areas of the body can be subjected to microwave or laser energy causing the lipid vesicles to rupture. One example of releasing drugs or other encapsulants by subjecting a designated area of the body to laser energy to rupture the encapsulant is disclosed in U.S. Pat. No. 4,891,043 to Zeimer et al.
The previous methods of rupturing the lipid vesicles to release the dye or other encapsulant have some drawbacks. First, microwaves and some wavelengths of laser energy tend to heat additional areas surrounding the lipid vesicles and may damage the tissue. Second, unless the lipid vesicles rupture simultaneously, the dye or encapsulant becomes diluted in the bloodstream. Examples of methods of measuring retinal blood flow using fluorescent dyes are disclosed in Hickam et al., A Photographic Method for Measuring the Mean Retinal Circulation Time Using Fluorescein, Investigative Ophthalmology, Vol. 4, No. 5, pp. 876-84, relating to laser Doppler velocimetry; Fallon et al., Measurement of Autoregulation of Retinal Blood Flow Using the Blue Field Entoptic Phenomenon, Trans. Ophthalmol. Soc. UK, Vol. 104, pp. 857-60, 1985, relating to the study of the leukocyte flow velocities in perimacular capillaries and the blue field entopic phenomenon; Greene et al., Quantitative Television Fluoroangiography--The Optical Measurement of Dye Concentrations and Estimation of Retinal Blood Flow, IEEE Transactions on Biomedical Engineering, Vol. BME-32, No. 6, pp. 402-06, June 1985, relating to video fluorescein angiography; and Khoobehi et al., Measurement of Retinal Blood Velocity and Flow Rate in Primates Using a Liposome Dye System, Ophthalmology, Vol. 95, No. 6, pp. 905-12, June 1989.
Numerous devices have also been developed to observe the fundus of the eye. Many of these fundus cameras require high light intensities which result in ocular damage. To overcome the risk of damage caused by high intensity light, laser scanning techniques have been employed. An example of a digital laser scanning fundus camera is described in Plesch et al., Digital Laser Scanning Fundus Camera, Applied Optics, Vol. 26, No. 8, pp. 1480-86, Apr. 15, 1987. This device uses a collimated laser beam focused by the eye to a spot of 10-15 microns diameter for illumination of a single point of the retina. The light scattered back from the retina, normally 3-5% of the incident light, is collected through the outer 95% of the pupil. Angular scanning of the illuminating laser beam sweep the spot across the retina and results in time resolved sequential imaging of the retina. The device is connected to a digital image buffer and a microcomputer for image storage and processing.
The above noted methods of introducing fluorescent dyes into the bloodstream have experienced some success in producing images of blood vessels and determining blood flow and blood velocity. These methods do not always produce accurate measurement of blood flow in the retinal macrocirculation and macular microcirculation. Furthermore, these methods are not able to measure blood flow in the small vessels of the optic nervehead. There is, therefore, a continuing need in the art for a method of directly and accurately measuring and detecting blood flow in the optic nervehead, retinal microcirculation around the macula, retinal macro circulation of the major vessel and choroid.